As the world's supply of conventional light crude is slowly diminished, challenging resources like the oil sands have become more economically attractive as sources of heavy, viscous oil also referred to as bitumen. Many methods for the production of heavy oil from geologic formations employ heated water and/or steam as the fluid medium to transfer heat to the targeted deposit, thereby reducing the viscosity of the hydrocarbon and allowing for more economic extraction of the heavy oil component. Two common thermal steam methods are Steam-Assisted Gravity Drainage (SAGD) and Cyclic Steam Stimulation (CSS). When steam or warm water is injected into a reservoir for this purpose, there is generally a significant amount of water carried along with the produced oil stream in the form of an emulsion. Water must be separated from this emulsion to yield relatively pure heavy oil or hydrocarbon for sale and pipelining. In many heavy oil operations, it is critical to further treat the separated produced water so that it may be recycled and reinjected downhole as liquid or steam to continue the heavy oil extraction process. In the specific case of SAGD operations, produced water must undergo significant treatment to ensure it is of sufficient quality (purity) for use as feed to once-through steam generators (OTSGs) or drum boilers typically used for the generation of steam for injection downhole into underground geological formations. Cyclic Steam Stimulation (CSS) is an example of another method for recovering heavy oil that involves injection of steam and treats the water stream produced in recovering the heavy oil. Whereas SAGD is a continuous injection process, CSS employs a cyclic process wherein the steam is injected and allowed to soak into the reservoir for a period of weeks or months before the well is switched to production.
A major concern with the use of heat exchangers for the heating and cooling of produced water streams in heavy oil processing is the propensity of these units to foul with various constituents of the treated stream. Hydrocarbon components such as bitumen and asphaltenes are notoriously tenacious foulants that adhere readily to tube walls and process equipment, thereby severely reducing the heat transfer capacity of the heat exchanger and, in many cases, restricting the maximum throughput of the entire water treatment system. The SAGD water treatment process in particular is prone to extreme fouling due to the high concentrations of asphaltenes, organic acids, scaling compounds and various dissolved or suspended solids that must be separated from the treated water stream before it can be reboiled to produce steam. Treatment of water in heavy oil operations is more fully explained in U.S. Pat. No. 7,077,201 B2 to Heins of GE Ionics, Inc.
One possible solution proposes to prevent fouling by coating the tube surfaces with epoxy coatings. However, such epoxy coatings have not been able to reduce extreme fouling.
SAGD operators have implemented numerous measures to mitigate the heat exchanger fouling problems with minimal success to date. Advanced cleaning methods, including chemical injection, heat sweeping and flow pulsing have been employed with little financial or productive benefit, while alternate exchanger designs (i.e., helically baffled, spiral-wound, etc) have also been employed more recently. Initial indications are that these alternate designs have little impact on the fouling rates observed at SAGD sites.
In WO 2009/014596 to Chakrabarty to Exxon Mobil Upstream Research Company, the problem of fouling in vessels used in a paraffinic froth treatment process which extracts bitumen from mined oil sands is discussed. Although fluorocarbon polymers adhered to the surface of a vessel or conduit are described as producing a reduction in the fouling observed on the surface, surfaces of molded PTFE are exemplified while sprayed fluoropolymers, especially those containing additives or fillers, were found to be ineffective.